The present invention relates to hydrophobic, surface modified, inorganic metal oxide pigments, such as titanium dioxide (TiO.sub.2) pigments, which are substantially free of aldehydes and other potentially volatile organic compounds on their surface. The invention also relates to an improved, environmentally safer method for the preparation of such pigments and to polymers containing such pigments.
Titanium dioxide is the premier white pigment used for whitening, brightening and opacifying paper, paints and plastics. As normally produced, TiO.sub.2 is a hydrophilic pigment, meaning that it is readily wet by water and not wet by hydrophobic materials like organic polymers. In order to permit TiO.sub.2 pigments to be wet-out by and dispersed in organic polymers, the surface of the pigment must be modified, or made hydrophobic, so that the polymer will spread over the pigment's surface and good adhesion between the pigment and polymer will occur.
Prior art references teach the preparation of hydrophobic TiO.sub.2 pigments by treatment with "non-reactive" organic substances, such as polydimethylsiloxanes (see e.g., Noll, Chemie und Technologie der Silicon, 2nd ed., 1968, page 386 et seq.), polyorganosiloxanes (see e.g., U.S. Pat. No. 4,810,305) and phosphorylated fatty acid derivatives (see e.g., U.S. Pat. No. 4,209,430). These prior art, non-reactive organic substances interact with the metal oxide's surface partially or completely through Van Der Waals forces and/or electrostatic interactions. Since these forces are comparatively weak, pigments treated with these organic substances may lose the coatings in later processing stages or the organic substances may be extracted from the pigments during use.
The use of "reactive" organic compounds to modify the surface of metal oxide pigments is also well known. U.S. Pat. Nos. 4,061,503 and 4,151,154 (both assigned to Union Carbide) disclose reactions of organosilanes with TiO.sub.2 to produce hydrophobic TiO.sub.2 pigments which enhance dispersibility in polymer matrices such as paints and plastics. In these patents the TiO.sub.2 surface is treated with a silane possessing at least two hydrolyzable groups bonded to the silicon and an organic group containing a polyalkylene oxide group. The hydrolyzable groups are described as alkoxys, such as methoxy and ethoxy. More specifically, U.S. Pat. No. 4,061,503 (which issued Dec. 6, 1977) describes the use of a polyethyl substituted silicon compound having alkoxy-containing hydrolyzable groups with from about 1 to about 4 carbon atoms. U.S. Pat. No. 4,151,154 (which issued Apr. 24, 1979) also discloses the treatment of titanium dioxide pigments with organosilicon compounds to improve dispersibility in polymers, similar to the '503 Patent, except its claims are directed to inorganic oxide particles generally and not just titanium dioxide.
European Patent Application No. 492,223 (published Jul. 1, 1992) discloses the treatment of TiO.sub.2 pigment with an organosilicon compound having the formula R.sup.1 R.sup.2 R.sup.3 R.sup.4 Si wherein R.sup.1 is a halogen or an alkoxy radical with 1 to 10 carbon atoms, R.sup.2 is an alkyl group with 1 to 30 carbons (preferably more than 8), and R.sup.3 and R.sup.4 are the same as either R.sup.1 or R.sup.2.
Great Britain Patent No. 1,154,835 (published Jun. 11, 1969) discloses a process for the treatment of finely divided materials, including titanium dioxide pigment. The patent indicates that inorganic powders may be rendered hydrophobic by treatment with organosilicon compounds. Specifically, the silicon compound has the formula R.sub.n SiX.sub.4-n wherein X is a halogen atom or a hydrolyzable alkoxy radical and R is a monovalent hydrocarbon radical (including an octyl [8 carbons] or an octadecyl [18 carbons] radical) and n has the value of either 0 or 1.
Suzuki, et al., "Chemical Surface Treatment of Alumina, Titania, and Talc and Their Respective Surface Properties," Shikizai, [J. Jap. Soc. Col. Mat.], Vol. 65, No. 2, pp. 59-67, 1992, describes the surface treatment of titania (large titanium dioxide crystals). As a comparison, the article refers to titanium dioxide particles that have been treated with octadecyltriethoxysilane--[the nonhydrolyzable group is octadecyl (18 carbons); the hydrolyzable groups are ethoxy]--to improve the particles' dispersibility properties in organic solutions and solvents.
Union Carbide's A-137 Product Information brochure (copyrighted 1991) cites to a organosilane compound wherein the nonhydrolyzable group has 8 carbons and the hydrolyzable group is ethoxy.
Great Britain Patent 785,393 discloses the treatment of TiO.sub.2 pigment with organosilanes to improve uniformity of coloration and to reduce streaking in polymer matrices. Table 1 discloses the use of nonyltriethoxysilane (the nonhydrolyzable group has 9 carbon atoms; the hydrolyzable group is ethoxy) and the use of ethyltrichlorosilane (the nonhydrolyzable group is ethyl; the hydrolyzable group is chloro).
Great Britain Patent 825,404 discloses a treatment of TiO.sub.2 pigment to improve dispersibility in organic solvents, including paint. In the patent, the organosilanes are represented by the formula R.sub.4-n Si(OR.sup.1).sub.n wherein R and R.sup.1 represent alkyl, aryl, or a hydrogen group. The preferred compounds include dimethyl-diethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, and phenyltriethoxysilane.
U.S. Pat. No. 4,141,751 discloses the treatment of TiO.sub.2 pigment with one of three different agents to improve the pigment's dispersion properties in various polymers. In one embodiment, the agent is R-Si-X.sub.3 wherein R can be an aliphatic or cycloaliphatic and X is a halogen or an alkoxy group. Preferably, the treating agent is a methyltrimethoxysilane.
Recently, several PCT patent applications by DuPont have published in which organosilanes, similar to those disclosed in the above cited references, are used for surface treatment of TiO.sub.2 pigment. For example, PCT patent publication WO 95/23192, published Aug. 31, 1995, discloses polymer matrices containing silanized TiO.sub.2 pigments in which a coating on the pigments contains an organosilicon compound having the formula: EQU R.sub.x Si(R.sup.1).sub.4-x,
wherein R is a nonhydrolyzable aliphatic, cycloaliphatic or aromatic group having 8 to 20 carbon atoms; R.sup.1 is a group selected from alkoxy, halogen, acetoxy or hydroxy or mixtures thereof; and x=1 to 3. Although halogens are mentioned as suitable hydrolyzable groups, all examples and preferred embodiments specify alkoxy groups. Specifically, the publication discusses the use of octyltriethoxysilane--[the nonhydrolyzable group is octyl or 8 carbons; the hydrolyzable group is ethoxy]. As obvious from the above discussion of the prior art, the DuPont patent application repeats various teachings already disclosed in the Union Carbide patents and the other documents cited above.
DuPont's application WO 95/23194 discloses a process for preparing silanized titanium dioxide pigments by media milling in which an aqueous slurry of the pigment is adjusted to pH 7.5 to 11, then treated with an organosilicon reagent. The reagent is essentially the same as that specified in WO 95/23192, and preferably one which contains an alkoxy hydrolyzable group, such as methoxy or ethoxy.
DuPont's application WO 95/23195 discloses titanium dioxide pigments which are treated with organosilicon compounds and boric acid or boron oxide. The boron ingredient can be dissolved in the organosilicon compound and the admixture applied to the pigments by dry mixing or in an aqueous slurry. The organosilicon compound again preferably contains an hydrolyzable alkoxy group.
As can be seen from the above discussion, organoalkoxysilanes have been traditionally used in the prior art for hydrophobizing inorganic pigments, such as TiO.sub.2. A major deficiency of procedures that use alkoxy silanes is the generation of volatile organic compounds (VOC's), such as methanol or ethanol, during hydrolysis of the silanes, according to the equation: EQU R-Si(OCH.sub.2 CH.sub.3).sub.3 +3H.sub.2 O.fwdarw.R-Si(OH).sub.3 +3CH.sub.3 CH.sub.2 OH
Producers of minerals treated with organoalkoxysilanes are obligated to protect their workers and the environment by collecting and disposing of these volatile organic compounds, which can often be expensive and time-consuming. A further deficiency of utilizing alkoxyorganosilanes is that under conditions encountered during processing and finishing the treated pigments, a portion of the volatile alcohol can be converted to noxious aldehydes. For example, ethanol can be oxidized to acetaldehyde which can remain on the surface of the inorganic oxide and be subject to evolution during further processing of the inorganic oxide by the inorganic oxide producer or customer. Further, alkoxyorganosilanes are among the most expensive organic materials known for hydrophobizing pigment surfaces.
Organohalosilanes are alternatives to organoalkoxysilanes for the treatment of inorganic oxide pigments. However, since these compounds, particularly the organohalosilanes, react vigorously with moisture and water, it would be expected that these reagents would have to be applied to the inorganic oxides dry or using nonaqueous, organic media.